The present invention relates to an image forming apparatus based on electrophotographic technology comprising a charging device.
In an image forming apparatus for forming an image based on electrophotographic technology, a photoconductor as an image carrier is provided with uniform charging by a corona discharge type charging device as a charging means, and the image is exposed by an image exposure means to form a latent image. Then a toner image is created by a development means, and a toner image is transferred onto a recording medium by a transfer device.
The corona charge type charging device used in the image forming apparatus of this type can be broadly classified into two types; a wire discharge type and a pin discharge type (pin electrode type and serrated electrode type). A grid for controlling the potential of the photoconductor is arranged between a discharge member such as a wire and pin, and a photoconductor.
When the photoconductor is exposed to a discharged product such as ozone generated by corona discharge, the photoconductor is subjected to deterioration. Even when subjected to exposure, the potential of the photoconductor remains close to the level of discharge potential without dropping to a predetermined level. Thus, an image is not formed, and a white streak in the horizontal direction (axial direction of the photoconductor) appears in the image.
Such a phenomenon tends to occur particularly when image formation operation is restarted some time after completion of the previous image formation operation. This is because ozone and other discharge products remain in the charging device and these remnants spread over the surface of the photoconductor, with the result that the photoconductor is deteriorated.
To prevent such deterioration of the photoconductor, the following methods are known in the prior art, for example; a technique of removing ozone and others by blowing air flow into the charging device (Patent Document 1), and a technique of rotating the photoconductor a predetermined time after completion of printing, so that ozone and others are removed by the flow of air during the rotation of the photoconductor (Patent Document 2).
[Patent Document 1] Official Gazette of Japanese Patent Tokkaihei 5-313470
[Patent Document 2] Official Gazette of Japanese Patent Tokkaihei 4-104267
Either of the aforementioned prior art techniques has failed to solve the problem of photoconductor deterioration.
Through concentrated study efforts to analyze the cause of this problem, the present inventors have found out the following:
A wire grid using a stainless steel or tungsten, or a plate-formed grid with a pattern formed on a sheet metal of stainless steel and others by etching is utilized as a grid arranged between the photoconductor and discharge member to control the charging potential of the photoconductor. The grid in particular has a large surface area and is characterized by powerful adhesion of ozone and other discharge products on such a material as a stainless steel or tungsten. Ozone and other discharge products attached onto the grid surface cannot be removed sufficiently by flow of air by rotation. When the image formation operation is stopped thereafter, ozone and other discharge products will spread gradually over the surface of the photoconductor, with the result that the photoconductor deteriorates. In particular, the grid is arranged close to the photoconductor, and this influence is very serious.
Especially in recent years, there is an active demand for downsizing and high speed processing of the aforementioned image forming apparatus. To meet this requirement, it is necessary to reduce the diameter of the photoconductor drum, to minimize the dimensions of a corona charging device, image exposure means development means and others arranged in the peripheral area, and to lessen the space between processing means. Further, when a photoconductor drums are arranged for each color, the space between these photoconductor drums must be minimized. This arrangement reduces the space of the image forming apparatus as a whole, and ozone and other discharge products generated from the charging device remain around the photoconductor without being removed. A white streak appearing in the image resulting from deterioration of the photoconductor presents a big problem for downsizing and high speed processing of the apparatus.